[libc++] Add a script to automatize updating test for a new header.

Idea from D92525.
This script globs include/ directory and updates the tests in test/libcxx.
This patch does not generate module.modulemap nor CMakeLists.txt.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D92656

libcxx/docs/Contributing.rst

@@ -23,18 +23,19 @@ After branching for an LLVM release:
 2. Update the ``include/__libcpp_version`` file
 3. Update the version number in ``docs/conf.py``
 
+Modifying feature test macros
+=============================
+
+When adding or updating feature test macros, you should update the corresponding tests.
+To do that, modify ``feature_test_macros`` table in the script ``utils/generate_feature_test_macro_components.py``, run it, and commit updated files.
+
 Adding a new header TODO
 ========================
 
 When adding a new header to libc++:
 
 1. Add a test under ``test/libcxx`` that the new header defines ``_LIBCPP_VERSION``. See ``test/libcxx/algorithms/version.pass.cpp`` for an example.
-2. Update the following test files to include the new header:
-
-  * ``test/libcxx/double_include.sh.cpp``
-  * ``test/libcxx/min_max_macros.compile.pass.cpp``
-  * ``test/libcxx/no_assert_include.compile.pass.cpp``
-
+2. Run ``python utils/generate_header_tests.py``, verify and commit the modifications.
 3. Create a submodule in ``include/module.modulemap`` for the new header.
 4. Update the ``include/CMakeLists.txt`` file to include the new header.